A frame part of an automobile is a press-formed product having a hat-shaped or groove-shaped cross section in many cases. The press-formed product includes a top plate section and two vertical wall sections. In an exact sense, a press-formed product having a hat-shaped cross section further includes two flange sections. Press-formed products have a variety of shapes. An example of a press-formed product having a complicated shape is a press-formed product including a top plate section that rises and falls along the longitudinal direction thereof. The press-formed product is configured such that the top plate section has a concave area curved inward in a side view and a convex area curved outward in the side view with the concave and convex areas continuous with each other in the longitudinal direction of the press-formed product.
To produce a press-formed product having such a complicated shape, a soft steel plate or a 440 MPa-class high-tension steel plate (these plates are hereinafter also collectively referred to as a “low-strength steel plate”) is mainly used as a blank plate. The tensile strength (TS) of a 440 MPa-class high-tension steel plate is 440 MPa or more, and the yield strength (YP) thereof is about 350 MPa or more. When a low-strength blank steel plate is formed in press forming into a press-formed product having a complicated shape, occurrence of wrinkles is a concern. Conventional press forming is therefore primarily draw forming (see WO 2014/042067 (Patent Literature 1), for example). A reason for this is that occurrence of wrinkles can be avoided in draw forming because the forming advances with tension applied to the blank plate. Bend forming for shape fixation is performed as the last process.
In recent years, an automobile is required to have improved fuel consumption from the viewpoint of global environment preservation. To this end, each frame part of an automobile is encouraged to have a reduced weight. From the circumstances described above, a 590 MPa-class high-tension steel plate (hereinafter also referred to as “mid-strength steel plate”) having a small plate thickness tends to be used as a blank plate of a press-formed product used as a frame part. Further, a 980 MPa-class high-tension steel plate (hereinafter also referred to as “high-strength steel plate”) having a smaller plate thickness tends to be used as the blank plate. The tensile strength (TS) of a 590 MPa-class high-tension steel plate is 590 MPa or more, and the yield strength (YP) thereof is about 400 MPa or more. The tensile strength (TS) of a 980 MPa-class high-tension steel plate is 980 MPa or more, and the yield strength (YP) thereof is about 600 MPa or more.
However, when a press-formed product having a complicated shape is formed in press forming from a mid-strength blank steel plate, and the press forming is primarily draw forming as described above, excessive tension occurs in some cases in the vicinity of a ridge present between the convex area of the top plate section and each of the vertical wall sections. The plate thickness of the blank plate therefore decreases in the vicinity of the ridges, resulting in breakage of the press-formed product in some cases. Further, compressive stress in the longitudinal direction of the blank plate is induced in the concave area of the top plate section. The material in the vicinity of the concave area of the top plate section therefore gathers, resulting in occurrence of wrinkles.
If the primary part of the press forming is replaced with bend forming using a pad, an excessive decrease in the plate thickness in the vicinity of the ridges adjacent to the convex area of the top plate section is avoided. Wrinkles, however, occur in convex-correspondence vertical wall areas (part of vertical wall sections) extending from the convex area of the top plate section and the concave area of the top plate section. In the case of a hat-shaped press-formed product, wrinkles occur also in convex-correspondence flange areas (part of flange sections) extending from the convex-correspondence vertical wall areas. A reason for this is that in the press forming, the material of the vertical wall sections and the flange sections that are not restricted by the press tooling moves toward the convex area.
That is, it is difficult in typical draw forming and bend forming to form a press-formed product having a complicated shape. Such a situation occurs in a more prominent manner in the case where a high-strength steel plate is used as the blank plate. A reason for this is that the ductility of a high-strength steel plate is further smaller than the ductility of a mid-strength steel plate.